Spray slow cooker with non stick spray. In a bowl, mix spinach, artichoke, sour cream, mayo, onion, garlic, mozzarella, Parmesan, garlic salt and milk. Add salt and pepper. Dump into your slow cooker. Place cubed cream cheese on top of mixture, spread out as evenly as you can. Cook on high for 2 - 2.5 hours or until cooked through and cream cheese is melted. Stir. Keep on warm setting until eaten. Serve with crackers, bread or vegetables.